One thought that I would suggest is not buying into an older system that is USB2. My mid 2010 iMac is still capable of most things I want to do, photography related, large photo library, Raw conversions etc but keeping data on external drives is a bit slow. Yes the machine does have FW800 but that is/has gone the way of the Dodo.
When I bought it I got an upgraded processor and put 16 GB of RAM into it to extend its usefulness. I have thought about putting a SSD into it but the bottleneck would be external storage for photo libraries wiyh FW or USB 2.
Just a thought.